Radio location techniques have been widely used in many mobile transmitter location systems (e.g., Loran-C). There are three primary methods used to achieve the location of a remote unit within a commnunication system, namely the a) Time of Arrival (TOA), b) Time Difference of Arrival (TDOA) methods, and c) Angle of Arrival (AOA) methods.
TOA Method for Location: When the distances .rho..sub.1 and .rho..sub.2 between a remote unit and a pair of fixed base stations are known, the position of the remote unit may be computed by determining the point of intersection of two circles with radii .rho..sub.1 and .rho..sub.2, each centered at one of the fixed base stations. If an error exists in either or both distance determinations, the true position of the remote unit, P, will be displaced and observed to lie at the intersection P', of the perturbed radial arcs .rho..sub.11 and .rho..sub.12. A third base station is typically needed to unambiguously locate each remote unit at the unique point of intersection of the three circles. To accurately compute the remote unit location using this method, it is necessary to know precisely the instant the signal is emitted from the base stations and the instant it arrives at the remote unit. For this method to work properly, an accurate measurement of the total time delay along each of three remote unit to base station signal paths is necessary.
TDOA Method for Location: For this method of location, the observed time difference between pairs of signals arriving at the remote unit from three or more base stations are used to compute the location of the remote unit. The remote unit, by observing the time difference in arriving signals between base station pairs, can establish the hyperbolae or "lines of position" (LOPs'). In this method therefore, the location estimate can be obtained without knowledge of the absolute arrival time of the signal from each base station at the remote unit, only time differences in arrival are significant.
AOA Method for Location: AOA location is described in U.S. Pat. No. 4,636,796, RADIO DIRECTION FINDING SYSTEM, by Imazeki and U.S. Pat. No. 4,833,478, AUTOMATIC DIRECTION FINDER ANTENNA ARRAY, by Nossen. Both patents are incorporated by reference herein. According to such a method, the angle of arrival of a signal transmitted from a remote unit is determined by analyzing the amplitude (or phase) differences between multiple antennas at a base site. Antennas in one sector will receive the remote unit's signal at a different angle of arrival than antennas in other sectors. It is the difference in received amplitudes (or phases) of the remote unit's signal at various antennas that is utilized in determining the angle of arrival of the signal. A look-up table may be used to equate the measured amplitude.sub.-- (or phase) difference to an angle of arrival.
In order to improve the accuracy of Location Finding Equipment (LFE), the LFE must be continuously calibrated. Such calibration is needed due to the ever-changing environment in which the LFE operates. For example, continuously-changing Radio Frequency (RF) propagation effects such as multipath scattering and reflections require frequent LFE calibration to provide accurate remote unit location within the communication system. Prior-art methods for calibrating LFE utilize error correction data obtained from fixed calibration terminals existing at known locations within the communication system. As described in U.S. Pat. No. 5,327,144 "CELLULAR TELEPHONE LOCATION SYSTEM" by Stilp et al., these fixed calibration terminals are distributed in a grid pattern at known locations, covering the geographic area of the communication system. Calibration occurs by periodically locating the positions of the fixed calibration terminals and calibrating the LFE accordingly.
Although the prior-art calibration technique described by Stilp et al., does increase the accuracy of LFE, such a calibration technique is costly to implement due to the number of fixed calibration terminals that are needed. Additionally, because the fixed calibration terminals are distributed in a grid pattern, "holes" exist within the grid where no calibration data can be obtained. Because of this, a need exists for a method and apparatus for calibrating location finding equipment within a communication system that is less costly, and more accurate than priorart calibration techniques.